Advances in Micro-Electro-Mechanical Systems (MEMS) technology and Nano-Electro-Mechanical Systems (NEMS) technology promise to revolutionize commercial, defense, and industrial products by bringing together the computational capability of microelectronics with the perception and control capabilities of microsensors and microactuators, thereby enabling smart systems-on-a-chip to be mass-produced. The use of smart systems that can actively and autonomously sense and control their environments has far reaching implications for a tremendous number of future military, commercial, and industrial applications and promises significant benefits for the United States economy and its citizens. The real potential of MEMS and NEMS becomes fulfilled when these miniaturized sensors, actuators, and structures can be merged onto a common silicon substrate along with integrated circuits. While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical components are fabricated using compatible “micromachining” processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.
This vision of MEMS and NEMS, whereby micro-miniaturized sensors, actuators and microelectronics can be integrated onto a single microchip is expected to be a revolutionary technological breakthrough. This will enable the development of a new class of systems by augmenting the computational ability of microelectronics with the perception and control capabilities of micro- and nano-sensors and micro- and nano-actuators. Microelectronic integrated circuits can be thought of as the “brains” of a system, and MEMS and NEMS augments this decision-making capability with “eyes” and “arms”, to allow micro- and nano-systems to sense and control the environment. Sensors gather information from the environment through measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena. The electronics then process the information derived from the sensors and through some decision making capability direct the actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby controlling the environment for some desired outcome or purpose.
While this is the future trend of MEMS and NEMS technology, the present state-of-the-art is considerably more modest and usually involves a single discrete transducer element that is rarely integrated with microelectronics. Some notable exceptions are the inertial (accelerometers) sensors used for automotive markets as crash airbag sensors and the micro-actuator optical display technologies, which have gained market acceptance in television displays and projectors. However, in both cases, the manufacturers of these MEMS technologies struggled to successfully integrate MEMS with microelectronics, resorted to using existing in-house microelectronics capabilities, and were pursuing relatively high volume MEMS markets. Typically even the most advanced development programs seldom attempt to integrate the MEMS with microelectronics. Most often new development programs are focused on developing new and revolutionary discrete non-integrated MEMS devices. Nevertheless, MEMS technology is transitioning from the realm of device research to that of systems integration, which is necessary for this technology to be truly useful for most applications. Presently, the approach taken for system integration is to use a hybrid package assembly whereby a MEMS die is co-located with a microelectronics die. For some applications, particularly those where system performance is not critical, this may be the best approach. But for many applications where high performance is critically important, size and power consumption must be minimized, and reliability must be exceptional, system integration necessitates that the MEMS be co-located with integrated circuits.
Many technologies, including, but not limited to, microelectronics, MEMS, NEMS, photonics, nanotechnology, employ thin film deposition as an integral part of the fabrication process, particularly technologies that use micro- and nano-fabrication techniques in their implementation. For example, many MEMS and NEMS devices, particularly those made using a category of fabrication called “surface micromachining,” use one or more deposited thin films as the structural or active layer of the device. As a result, thin films are used where the properties, particularly mechanical properties, of the materials matter the most. For many MEMS and NEMS devices having a mechanical functionality, perhaps the most important material properties are the residual stress and stress gradient. Importantly, the stress state of MEMS and NEMS structural layers will have a huge impact on the resultant device's behavior and performance, and as such, should be accurately known and well controlled in device implementation. Ideally, these material properties should also be able to be tailored, or adjustable, over a range of values specific to the device performance and application requirements.
Currently used thin film deposition techniques almost always introduce significant residual stress and stress gradients in thin film layers. Moreover, using existing deposition techniques, the stress state has typically been difficult to either predict or control, especially in devices made from multilayer thin film structures.
Present understanding of the relationship between states of residual stress and the processing conditions through which thin films are deposited is typically very poor and at best highly qualitative. Consequently, it is extremely difficult to design and fabricate devices if the mechanical properties of the materials used are not known, able to be controlled, and cannot be deposited reproducibly.
Furthermore, uncontrolled residual stresses can induce a variety of highly undesirable consequences, and include, but are not limited to, cracking, delamination, deformation, and microstructural changes in the materials. The need to ensure thin film mechanical integrity through controlled stress states continues to be a technology limiting factor for the implementation of MEMS, NEMS and other devices and is not merely limited to applications where load carrying capacity is the sole function.
Another limitation of deposited thin films for use in micro- and nano-fabrication is that they are often too thin for many device applications, particularly those in MEMS technology applications. MEMS devices often require very high aspect ratios and thicker films in order to meet specific device performance requirements. Unfortunately, the high stress states of many deposited material layers preclude the deposition of thicker film layers (e.g., not more than 1 to 2 microns for some materials and not more than a few thousand Angstroms for some others). Adequate control of thin film stress in situ can remove these thickness limitations and allow extremely thick films, even layers more than 100 μm in thickness, to be deposited. Additionally, stress-related adhesion issues that currently plague certain useful materials (i.e., Pt, Ni, Mo, MgF2) may be overcome by stress-minimization and enhanced adhesion from increased energy transfer during initial the stages of film growth.
Currently, there is extremely limited ability to control the stress states of deposited thin films and this is a severe challenge for micro- and nano-fabrication technologies. As a result, device development is more costly and time consuming, production yields are lower resulting in higher manufacturing costs, and the performance of devices will be limited. It also limits the thickness of the layers that can be deposited, thereby restricting the design and process freedom for device implementation.
There is a need for improved methods for the deposition of material layers wherein the stress can be tailored to the specific device design and application. The benefits of controlling the stress in deposited thin films for micro- and nano-device fabrication is truly enormous. The ability to make thin films having predictable, controlled and reproducible stress levels allows device designs to be implemented much faster and more inexpensively. Further, it will allow thicker films made by PVD deposition techniques, thereby providing far greater design latitude. Thin film stress control makes possible a near-zero stress film or stack of films, which is extremely useful for most micro- and nano-device applications. In short, deterministic stress control of deposited films is truly a useful and much needed technology development.
There is a need to be able to fabricate MEMS and NEMS on a substrate that has had previous microelectronics processing performed on it, such as CMOS processing, and it is desired to perform further processing on the microelectronics wafer to enhance the devices or add new devices, such as MEMS or NEMS devices, so as to integrate the MEMS or NEMS with electronics onto the same substrate. The temperature of substrates with pre-existing microelectronics will have a temperature constraint placed on them whereby the substrates cannot be exposed to temperatures above 400 to 450 C for any significant amount of time, typically only a few minutes, before the pre-existing microelectronics have been degraded. There is a need for the fabrication of integrated MEMS and NEMS to obtain a low stress deposition at a sufficiently low deposition temperature for various thin films for the fabrication of MEMS and NEMS devices directly onto the substrate with pre-existing microelectronics.
Current thin film deposition techniques used in micro- and nano-fabrication are open-loop systems. That is, there is no mechanism for determining material properties until after the deposition is completed and measurements are subsequently taken on the sample. Closed-loop feedback control of thin film deposition would provide in situ stress control, as well as repeatability and consistency of the material that was deposited. Pressure and deposition rate monitoring are commonly equipped on deposition systems and utilized during thin film deposition, while stress measuring is absent. While thorough process development and frequent post-deposition characterization will initially produce stress-controlled thin films, the degree of control is less than that obtained with in situ monitoring. Additionally, closed-loop feedback stress control is immune to deposition and hardware drift, as it compensates the processing parameters in real-time. Therefore, closed-loop control of the stress in thin film deposition would be a useful invention.